edit: The 2SMPP-O2 ic is upside down, so it probably looks wrong. It only comes in a SMD and I had to drill a hole in a DIP adapter for the pressure port, and put it upside down. So it goes like this in pic:
1 2 3 -
6 5 4 -
Also, do you consider this very noisy now that I've zoomed in? Attached pdf.
So where there is voltage output of the sensor, I bridge it to ground with a capacitor?
And where there is voltage output coming out of the opamp, I bridge that to ground too with a capacitor?
If you are looking for a 2 terminal current source try a CC100 diode. It's a specially constructed Jfet that is a 2 terminal constant current diode (@ 100uA). It can be done with a Jfet and a resistor but I suspect that this is a little beyond your current grasp of theory. There are extensive design notes available in the App note but I doubt they are available any longer... Siliconix was one Mfr of CC diodes...
The app notes I refer to are from the National Semiconductor analog design app notes, Vol 2.
I think part of my noise problem is the resistor I'm using now for the instrumentation amplifier.
I don't know the tolerances on the one I have but it's probably not good. The one I was expecting to use, until it was apparent the output of the sensor was 10x what the datasheet said, was very good. Now I'll just need to find one with better tolerances.
I hooked up the resistor with lower tolerances and the signal looks pretty clean with me just blowing pressure in the tube to the sensor.
Words can be such a pain. Lower tolerance is the opposite of +/- smaller percent. Did you really mean that using a lower tolerance resistor made it work when the higher tolerance resistor did not? If so then the resistance you really want is not the one calculated but closer to what the LT resistor actually is.
A 5% tolerance resistor is higher tolerance than a 10% tolerance resistor. The higher the tolerance, the higher the accuracy must be to pass. It's intuitive if you make things to tolerances but otherwise not.
I think that they test the things every so many and if within 10% then that part of the run goes in the 10% bin even as the process is tweaked for higher accuracy. Still there's nothing saying that +/- 10% can't be within 1%, just that's not likely given consistent manufacture.
The signal is looking better with a better resistor for the instrumentation amplifier. I used a 10.2kOhm/0.1% tolerance. I attached a pdf of the pressure signals again.
The pictures are from 2 different trials.
Oddly, it does seem the device is altering the pressure a bit when it is detecting the systolic & diastolic blood pressure. Cause some noise-like signal appears where those pressures are in each trial.
Don't trimpots usually have much higher tolerances and produce noisier signals?
Tolerance? They adjust. Noisy? I don't know, mine don't seem to be. Maybe they are -while- they're being turned. Otherwise get audio pots. The whole idea is to be able to tweak the resistance instead of picking the resistor closest to best, since they only make so many values of resistor.
I guess you could also put some fixed-value resistors in parallel to get a between value too. It'd be cheaper and noise-free.
oscarcar:
Don't trimpots usually have much higher tolerances and produce noisier signals?
Trim pots of the 10 and 20 turn variety are often used in precision high quality instrumentation circuits for calibration purposes. One trick is to not let the pot control a larger range then it needs to be, so if you need a +/- 5 percent range of adjustments you might use say a 100 ohm 20 turn trim pot that has it's end terminals wired to 1% 10K fixed resistors. That way any drift in the pot due to tempco would have a much smaller effect in the overall circuit compared to just using a 20K ohm trim pot.
Is it really necessary for you to be able to tweek the gain of your instrumentation amplifier? It seems to me you just need to select a precision resistor that gives you the gain you need for the maximum signal level that doesn't exceed your ADC input range. Use the instrumentation amp's gain equation to get the actual gain that results from the value of the gain resistor you actually use.
I imagine you are balancing the transducer at zero pressure in software. I don't think you've talked about how you intend to calibrate it. Maybe the data sheet span voltage is good enough for your application, or maybe the transducer comes with a calibration sheet.
t seems to me you just need to select a precision resistor that gives you the gain you need for the maximum signal level that doesn't exceed your ADC input range.
t seems to me you just need to select a precision resistor that gives you the gain you need for the maximum signal level that doesn't exceed your ADC input range.
So you can get non-standard value resistors?
It seems to me you just need to select a precision resistor that gives you the gain you need for the maximum signal level that doesn't exceed your ADC input range. Use the instrumentation amp's gain equation to get the actual gain that results from the value of the gain resistor you actually use.
Let me state it more simply. Decide what maximum gain you need, based on not exceeding your ADC's input spec. Solve the data sheet equation for the resistor value that will give you that gain. Select the standard value of the precision (1%, 0.1%, 0.05%, etc) resistor that is closest to the calculated value without causing the amplifier to exceed the maximum gain. Then, if you need the actual amplifier gain in your calculations, solve the data sheet equation for gain based on the resistor value you actually use. Is that more clear?
oscarcar:
Don't trimpots usually have much higher tolerances and produce noisier signals?
Trim pots of the 10 and 20 turn variety are often used in precision high quality instrumentation circuits for calibration purposes. One trick is to not let the pot control a larger range then it needs to be, so if you need a +/- 5 percent range of adjustments you might use say a 100 ohm 20 turn trim pot that has it's end terminals wired to 1% 10K fixed resistors. That way any drift in the pot due to tempco would have a much smaller effect in the overall circuit compared to just using a 20K ohm trim pot.
Lefty
Quite true. Also as microcomputers and SOC's get cheaper and cheaper, nowadays it is becoming quite common to use a fixed resistor and do the calibration in software during final test of the instrument. When it's time for a periodic instrument calibration, if anything changes, the calibration values are simply updated in EEPROM or whatever.
I always got it. Calibration/tweaking can be done in software or externally via hardware adjustments. Depends on the application and users requirements. If it's for one's own project then software is cheaper as one has the freedom of recompiling the sketch at any time when external things are changed. If however the project is designed to support changes in sensors or other external changes by users of the project then external calibration might be more desired. At the refinery I worked at we used thousands of sensors wired into central control systems, and we always required that any sensors used had the means to calibrate them externally so that we did not have to have 'custom' loop calibrations for every sensor wired to the central systems.
Different strokes for different folks, there is no single "best method", just what works best for you.
I always got it. Calibration/tweaking can be done in software or externally via hardware adjustments. Depends on the application and users requirements. If it's for one's own project then software is cheaper as one has the freedom of recompiling the sketch at any time when external things are changed. If however the project is designed to support changes in sensors or other external changes by users of the project then external calibration might be more desired. At the refinery I worked at we used thousands of sensors wired into central control systems, and we always required that any sensors used had the means to calibrate them externally so that we did not have to have 'custom' loop calibrations for every sensor wired to the central systems.
Different strokes for different folks, there is no single "best method", just what works best for you.
Lefty
When I first read this thread, I wondered if I had the means to calibrate a 0 - 37 kPa transducer. It turns out my smaller Chandler deadweight tester will actually go down that low. I wouldn't be able to match the exact full scale, but 1 psi increments would probably give a good enough calibration.